The present invention relates to fasteners, and particularly ones which are used in the aircraft industry to affix external components and skin to airframes. The inventors have developed a system for applying evenly distributed torque along the entire length of a threaded fastener used to hold down aircraft components. The present system will be particularly advantageous over current systems for removal of fasteners frozen in place due to corrosion, deformation, paint, or wear.
Modern aircraft are constructed, in some respects, as they have always been, a veneer or skin is stretched over and fastened to a frame which then forms the fuselage, airfoils, and other aerodynamic surfaces. Most commonly, the veneer is an aluminum alloy, and the fasteners are stainless steel. During initial assembly, the fasteners are tightened using one of several common head drive mechanisms such as the Phillips, TorqSet, Torx, or other drive mechanism with a matched fastener head. Technicians, however, are often required to remove the fasteners during the normal life of the aircraft for routine maintenance, to perform repairs, or to modify or upgrade the aircraft. The fasteners, during the removal process, are frequently damaged and, in some cases, must be drilled out and removed with a specially designed screw extractor. This can be extremely costly, time consuming, and often cause damage to the panel or adjacent structure. This problem has been documented a report issued by the U.S. Air Force, titled AF95-203 "Improved Recess Drive System for Fasteners".
While attempting to remove fasteners, torque applied only at the head of the fastener may be insufficient to effect removal for a variety of reasons: axis of torque application and screw center not coincident, cam out (stripping) of head, or dried paint under the screw head or in the drive faces. Also, if the screw threads are tightly stuck, the shear stresses where the head meets the screw body can often exceed the maximum allowable stresses and result in screw failure. Increasing the drive area, as the present invention does, reduces the pressure applied to the drive surfaces for a given torque. When a screw is drilled out for removal using an extractor tool, the increased surface area is what allows use of higher torque overall without metal deformation.
Another benefit of the present invention, especially for larger bolts, is the possibility of integrating strain gauges along an interior wall of the fastener for accurate monitoring of preload tension and torsion on the fastener. This could help eliminate many of the bolt failures which, due to the torsion force vector in addition to the tensile force vector during initial tightening, are most likely to occur upon installation. Integrated strain gauges could be of particular significance in larger bolts, especially those used in the construction of buildings, bridges, and vessels.
The problem of removing fasteners, frozen in place through oxidation, deformation, or as a result of excess paint and allow maintenance or replacement of aircraft components has presented a challenge to designers. The development of a method, other than drilling out the fastener, for removing frozen fasteners would satisfy a long felt need in the field of aircraft maintenance and modification.